This invention relates to the making of nickel positive electrodes for secondary, alkaline storage batteries, and more particularly to a unique method of distributing and supporting fine particles of nickel hydroxide and graphite throughout the electrode so as to achieve maximum utilization of the nickel hydroxide, and produce an active mass particularly useful with a lightweight grid/conductive support.
One of the major drawbacks to the more extensive use of nickel alkaline batteries (e.g., nickel-cadmium, nickel-zinc, etc.) is the high cost of the nickel (i.e., nickel hydroxide) positive electrodes. Originally such electrodes utilized porous nickel plaques of sintered carbonyl nickel powder impregnated with nickel salts which were then converted into nickel hydroxide. Typically this was accomplished by filling the pores of the nickel plaque with an aqueous solution of a nickel salt and subsequently converting the salt to the hydroxide by chemical, electrochemical or thermal processes. The process normally required several repetitions to introduce the desired amount of nickel hydroxide into the plaque and utilized unnecessarily high amounts of nickel which added considerable cost and weight to the battery.
Later developed electrodes eliminate the expensive nickel plaques. Some are made by milling (i.e., calendaring) nickel hydroxide, graphite, binder and a plasticizer together and then roll bonding it to a current collecting grid. Porosity is obtained in these electrodes by various techniques. In one case, a mixture of two immiscible thermoplastic resins is used as the initial binder, and later one of the resins is leached from the mass with a suitable solvent. The active electrode material is retained and bound in a microporous matrix of the insoluble thermoplastic resin. An additional sintering step may be employed to remove any remaining soluble resin. While materials-wise this technique is less expensive than the sintered plaque electrode, manufacturing-wise it was still quite involved and produces electrodes with the utilization efficiencies [i.e., ampere-hrs/gram of Ni(OH).sub.2 ] less [i.e., about 0.23-0.24 A.sup.. h/g Ni(OH).sub.2 ] than are obtained with the present invention.
Still other proposed techniques include: (1) percipitating nickel hydroxide as a slurry from a solution of a nickel salt and vacuum impregnating a porous nickel conductor with the slurry; (2) applying a layer of an aqueous paste of nickel hydroxide, nickel powder and a binder to a metallic substrate, compressing it to remove excess water, drying it and compressing it again to achieve intimate nickel hydroxide-nickel metal interfacial contact; (3) mixing nickel hydroxide, graphite, dimethylformamide, polyvinylidene fluoride and dimethylacetamide together, casting it into a thin film (e.g., 0.7-0.8 mm), drying it for a short while, immersing it in water to coagulate the polyvinylidene fluoride and finally wrapping it with a current collector and fabric separator to form the electrode. Each of these processes are complex, time consuming and from available data appear to offer no advantages, utilization-efficiency-wise, over the solvent-extracted-resin technique.
It is therefore an object of this invention to provide a simple process for manufacturing efficient, lightweight, low cost nickel electrodes for secondary nickel alkaline storage batteries whereby nickel hydroxide and graphite particles are bound together in a three dimensional, open-celled polyvinylidene fluoride matrix so as to achieve maximum utilization of the nickel hydroxide. This and other objects of this invention will become more apparent from the detailed description which follows.